Ensure update to ReactiveData value sets correct old value on event

[philippjfr]

Fixes #3188

[codecov]

Codecov Report

Merging #3308 (eb839f8) into master (80d90ee) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3308      +/-   ##
==========================================
+ Coverage   83.39%   83.40%   +0.01%     
==========================================
  Files         196      196              
  Lines       26509    26509              
==========================================
+ Hits        22108    22111       +3     
+ Misses       4401     4398       -3     

Impacted Files	Coverage Δ
panel/reactive.py	77.60% <100.00%> (+0.22%)	⬆️
panel/widgets/tables.py	87.88% <0.00%> (+0.10%)	⬆️

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